Higher and higher precision is constantly required for either power supply circuits or level control circuits employing comparators or amplifiers. In all instances, it is of fundamental importance to establish a reference voltage upon which such functional circuits may be based. Such reference voltages should be highly stable with respect to temperature changes and to shifts of the supply voltage. Such reference voltages should also be essentially free from noise that may come from the supply lines. In addition, such circuits should be capable of delivering a sufficient current to an output load.
Many integrated circuits have been implemented in an attempt to obtain stable voltage references with a high PSRR. The common approach is to stabilize the supply voltage of a so-called bandgap cell, thus averting the Early effect of the bipolar junction transistors and or the body effect of field effect transistors used in the cell. These effects would otherwise cause slight variations in the output voltage provided by the bandgap cell. There are numerous articles on these topics although perhaps the most popular approach is based on the use of the so-called Brokaw cell. Mr. Barrie Gilbert of Analog Devices thoroughly described this approach during the "Low-power Low-Voltage Analog IC Design" workshop held in Lausanne in June 1996. Upon analyzing the basic circuit of the Brokaw cell, many approaches have been proposed and FIG. 1 shows a typical circuit implementation, which, in any case, has few drawbacks.
As shown in FIG. 3, by observing the evolution of the output voltage at start up, most of the circuits of the prior art show a characteristic along which two different slopes may be clearly identified. By referring to the scheme of FIG. 1, a bandgap cell (or Brokaw cell) requires a certain current to be activated. Therefore, a bipolar junction transistor (BJT) Q9 is normally present to force an adequate current through the cell circuit. However, the collectors of the transistors Q13 and Q11 initially are at null voltage, and this determines that if their base voltage increases, the associated pnp parasitic transistor is turned on. In this case, if the base bias current of these transistors, which originate from Q8 and Q9, is not greater than the current leaked toward the substrate by the parasitic pnp transistor, the npn pair will not come out of saturation and the bandgap cell will not start up. Instead, it will remain blocked at a voltage of about 0.6-0.7v, that is, at the Vbe of the parasitic pnp.
Of course, if the design is correct, the circuit will start up, but the output voltage will not increase linearly because the Vbg voltage has not exceeded 0.6V (that is, the previously cited critical value). The circuit is then only capable of supplying a small amount of current to the load capacitance, and, thus, the Vbg voltage on the associated node of the cell increases slowly. At a certain point, the pair of transistors of the bandgap cell turn on definitely and the voltage grows rapidly towards its final value. This is so because the output transistor of the Q8 cell also begins to deliver current to the load. This typical output voltage characteristic is depicted in FIG. 3.
It appears evident that besides the parasitic pnp transistor, an accidental cause for a missed start-up may be an excessive load on the circuit output during the start-up phase. This is so because the excessive load draws current from the bases of the npn pair of the cell, thus enhancing the undesired effect of their parasitic pnp transistor.